Detectors designed for infrared imaging are conventionally produced as a one or two-dimensional array of elementary detectors, or bolometers, said bolometers taking the form of membranes suspended above a substrate, which is generally made of silicon, by means of support arms that have a high thermal resistance. This assembly of suspended membranes forms an array detection element that is usually referred to as a “retina”.
The substrate usually incorporates means of sequentially addressing the elementary detectors and means of electrically exciting the elementary detectors and pre-processing the electrical signals generated by these bolometers. This substrate and the integrated means are commonly referred to as the “readout circuit”.
In order to obtain a scene by means of this detector, the scene is projected through suitable optics onto the retina which is arranged in the focal plane of the optics and clocked electrical stimuli are applied via the readout circuit to each of the bolometers or to each row of such bolometers in order to obtain an electrical signal that constitutes an image of the temperature reached by each of said elementary detectors. This signal is then processed to a greater or lesser extent by the readout circuit and then, if applicable, by an electronic device outside the package in order to generate a thermal image of the observed scene.
This type of detector has numerous advantages in terms of its manufacturing cost and implementation but also has drawbacks that limit the performance of systems that use such detectors. In particular, there are problems with regard to the uniformity of the image obtained. In fact, when exposed to a uniform scene, not all the bolometers deliver exactly the same signal and this results in fixed spatial noise in the image thus obtained and this noise has a seriously adverse effect on the quality of the images produced. It is commonly referred to as “offset dispersion”.
An image obtained from the array of bolometers is then corrected for offset dispersion by subtracting, from each pixel of said image, the offset that corresponds to that pixel which is stored in an “offset table” obtained during factory calibration by exposing the retina to a black body having a constant, known temperature. The corrected image of a uniform scene is then substantially uniform.
Nevertheless, determining an offset table is usually a tricky, time-consuming task. It actually involves presenting the detector with a scene having a known uniform temperature, classically a uniform-temperature black body, taking care to ensure a constant retina temperature that is substantially equal to that of the black body at the time of acquisition.
Also, the offset of a bolometer depends on its temperature so that if the temperature of the bolometer deviates from the temperature at which the offset table was determined, the latter becomes irrelevant and correction becomes unsatisfactory.
In order to overcome this problem in a first type of bolometric detector, the focal plane is temperature controlled, for example by means of a Peltier-effect module or a heater. The intention is to make sure that variations in the temperatures of bolometers are caused exclusively by radiation originating from the observed scene.
In a first version of a detector with a temperature-controlled focal plane, temperature regulation is performed based on a single predetermined temperature setpoint. This is referred to as a “single-temperature control mode”. This version has the advantage of substantially limiting the operating temperature range of the detector either side of said setpoint, thus making it possible to use a single offset table. This limits the extent of factory calibration. On the other hand, the detector's energy consumption is far from optimal and this may even become a limiting factor in the context of stand-alone portable detectors. Indeed, when the detector's ambient temperature deviates substantially from the temperature setpoint, the energy used for temperature control purposes is considerable. Not only that, even if temperature control is implemented, it cannot ensure a perfectly constant temperature. The detector is constantly subjected to temperature disturbances originating from the environment of the retina, especially the package in which it is fitted; the temperatures of this package and its associated elements (optical unit, diaphragm, etc.) change freely as a function of radiation and other interference originating from its external environment. In fact, there are always transients that deviate from the temperature setpoint and these transients become greater the more the ambient temperature deviates from the setpoint.
Because single-temperature control mode is energy-consuming and becomes increasingly less accurate the more the ambient temperature deviates from the focal plane's single temperature setpoint, a second version of temperature-controlled detectors referred to as “multi-temperature controlled detectors” uses several temperature setpoints so that the difference between ambient temperature and the temperature of the retina is kept below a predetermined threshold. This minimises the energy consumed for temperature control purposes and the inaccuracy of the offset correction is contained regardless of temperature. However, this presupposes having an offset table for each temperature setpoint. The quantity of offset tables is usually large in order to reap maximum benefit from the advantages of the multi-temperature control mode and this involves very protracted factory calibration of these tables and therefore considerable manufacturing costs. In addition, in use it is found that transitional phases when there is a change from a first temperature setpoint to a second temperature setpoint generally cause a loss in the quality of the images produced by the detector.
Because temperature-controlled detectors consume large amounts of energy and are cumbersome and heavy, non-temperature controlled detectors, or “uncooled” detectors, commonly referred to as TEC-less (Thermo-Electric Cooler-less) detectors have been developed.
In a first version of a TEC-less detector, for example that described in documents EP 1 953 509 and U.S. Pat. No. 6,433,333, a plurality of offset tables are acquired in the factory for various temperatures of the focal plane over the presumed operating range of the detector and are then stored in the detector. Usually, in order to set the detector's ambient temperature, it is placed in a thermostatted enclosure that holds each one of a series of steady temperature levels for approximately one hour. In the final analysis, this calibration process takes several hours and requires a thermostatted enclosure and is therefore particularly expensive for the manufacturer.
During operation of the detector, the temperature at one point on the substrate is measured and an offset table is selected from the stored tables as a function of the measured temperature or an operational offset table for the measured temperature is obtained by interpolating the stored offset tables. The offset table thus produced, and consequently the correction table, are therefore temperature dependent. However, the effectiveness of such a correction depends on the relevance of the offset table that is used. In fact, it is necessary to provide a considerable number of tables for the temperature range in question and this is expensive.
Because the use of offset tables has proved to be uneconomical, other types of correction have been designed.
In a second version of the TEC-less detectors, for example that described in documents U.S. Pat. No. 5,756,999 and U.S. Pat. No. 6,028,309, the offset dispersion of bolometers is corrected by applying a variable bias to them. Indeed, the signal output by a bolometer depends directly on the current that flows through it. Modifying this current therefore modifies the bolometer's continuous output level and hence the value of its offset. However, this type of correction involves using custom-built bias circuitry for each bolometer and this makes designing the circuits of detectors much more complex and reduces fabrication yields. Not only that, detrimental deterioration of the signal-to-noise ratio is also observed. Also, this type of correction still requires offset tables although in limited quantities compared with the number of tables required by the first version.
In a third version of the TEC-less detectors, for example that described in document U.S. Pat. No. 6,690,013, offset dispersion is corrected as a function of the measured resistances of the bolometers based on an empirical model. However, simply measuring the electrical resistances of the pixels is not representative of all the causes of offset dispersion. Correction performed in this way is therefore only partially effective. In addition, the empirical model still uses parameter tables that require factory calibration similar to the calibration involved in producing offset tables.
Finally, in a fourth version of the TEC-less detectors, for example that described in document WO 2007/106018, offset dispersion correction is based on the actual scene itself and, more especially, on using the temporal evolution of information in the scene observed by the detector. This type of correction has the advantage of not requiring any prior factory calibration. On the other hand, corrections of this type are unsuitable for moving scenes because, by their very principle, such corrections eliminate or at least seriously degrade the detection of static elements or slow-moving elements in the scene. In addition, “gost” images that are not representative of the observed scene can appear under certain conditions.
Document US-A-2005/0029453 discloses a method for updating an offset table on the basis of two images from a shutter taken while the detector is operating. This method involves testing a condition for updating an offset table, for instance a condition that relates to the temperature variation observed since the offset table was last computed or a condition that relates to the age of the offset table, and acquiring a new image from the shutter if the condition is met. Once the new image has been acquired, a new offset table is then computed as a function of this new image and the image acquired at the time of the last update.
It should be noted that corrections based on previously calibrated offset tables are the most effective because the values contained in said tables are directly linked to measured offsets of bolometers. In addition, such corrections do not involve any limits on operation of the detector. Alternative corrections that try and limit or even eliminate offset tables make it possible to design TEC-less detectors less expensively but have a detrimental impact on correction quality or impose limitations on use of the detector.